Since I'm the only one here with a future in writing quality literature (including @Killa_Robot as he doesn't sem to be making new stories), I will endow upon you the diegesis you so utterly require. You're welcome.
Before the Story starts:
The hero spent his life as the member of a rebellion, crusading against the corrupt empires of the age. These monarchies would recruit and enslave the most dreadful creatures of the land to do their bidding and fight their wars. In all of this, human life was threatened, so the hero single-handedly formulated a coalition to subdue these thrones and bring salvation to the people of these lands. He required the aid of the greatest warriors and those with the ability to tame the fearsome beasts. Time passed, allowing the hero's coalition to develop to a formidable army of the land's greatest fighters and monsters. Their battle force seemed unstoppable until the monarchies realized it was time to form together and crush this foe once and for all. The empires resolve their differences, agree to bind, and propose one consolidated force that, in an awe-inspiring battle for the ages, destroyed the hero's awesome regiment.
As the Story starts:
Wizards of the highest caliber were recruited to manipulate the mind of the leader that was the hero, but the resolve of the hero was too resilient. In the end, he managed to escape, but felt his memory slipping away. The beloved hero came to lose his memory and after a matter of decades, his name was banished from the empire, and his legacy forgotten. In a cruel twist of irony, he ended up joining the ranks of the empire's military as its reach expanded. He is now required to do the deeds (fighting monsters, collecting cash) he once fought to abolish.
The Main Arc of the Story:
Fortunately, there are those in the villages who still believe in this hero, however aged he may be, and as such, the faithful people one day take the burden of restoring him to his former glory. To do this, they hire the help of his former allies (the ones who have survived). Assassins were enlisted to kidnap him when convenient. Medics were hired with the intent to heal him after his battle. Warlocks were called upon to help him restore his memory. With enough help and faith, the people were sure they would receive their great savior again and be able to destroy the empire once and for all!
Use that as the starting basis for your story. And no, I will not be co-authoring your story.